1. Field of the Invention
The present invention relates to an improved heart valve prosthesis for use in the surgical repair of atrioventricular heart valves.
2. The Prior Art
The mitral and triscuspid valves are located in the atrioventricular openings of the heart and serve to prevent regurgitation of blood from the ventricle into the atrium when the ventricle contracts. The mitral valve is located in the left atrioventricular opening of the heart. It is encircled by a dense fibrous ring known as the annulus and consists of two valve cusps or leaflets. These two leaflets are of unequal size. The anterior leaflet, being situated adjacent to the aortic opening, is often called the ventral, anterior, or aortic cusp. The smaller posterior leaflet is known as the dorsal or posterior cusp. The leaflets are composed of strong fibrous tissue which is thick in the central part but thin and translucent near the margin. They are held in place by chordae tendinae and papillary muslces. The line at which the leaflets come together is called a commissure.
The tricuspid valve is located in the right atrioventricular opening and comprises three leaflets, sometimes referred to as the anterior, posterior, and septal cusps. These leaflets are roughly triangular in shape and are also attached to a fibrous ring.
The mitral valve is subjected to significantly higher back pressure than is the tricuspid valve. Accordingly, it is far more common to require surgery to repair a mitral valve than for a tricuspid valve.
Normal function of the mitral valve requires the coordinated interaction of six anatomic elements: the leaflets, chordae tendinae, annulus, the left atrium, the papillary muscles and the left ventricular wall. The most common defect leading to mitral dysfunction is a dilatation or elongation of the posterior two-thirds of the annulus, the section corresponding to the posterior leaflet. The anterior portion of the annulus is anchored to the aortic root and is thus not as subject to elongation. However, not infrequently in cases of mitral valve dysfunction the anterior leaflet is displaced away from the center of the valve and is slightly thickened and shortened. Thus, in repairing a mitral valve, it is sometimes necessary to reduce the annulus in its physiological dimensions by repairing the dilatated posterior two-thirds thereof. Also, it is generally necessary to restore the commissure to its normal physiological curvature. Also, it is often necessary to reposition and reshape the anterior leaflet to correct any malposition and shortening of tissue which may have occurred.
Although the discussion above has been directed specifically towards the mitral valve, similar concepts apply to the correction of tricuspid valve defects.
One solution to serious valve dysfunction is total valve replacement. However, it is generally agreed that repair of the valve by techniques of annuloplasty are preferable to valve replacement. One valve prosthesis is described in U.S. Pat. No. 3,656,185 to Carpentier. This prosthesis consists of a rigid annular or part-annular member adapted to fit against the base of the valve leaflets and secured in place by sutures. Although this device constituted a significant advancement in the treatment of mitral valve dysfunction, because of its rigidity it did not allow natural movement of the annulus during the cardiac cycle. In a normal heart, there are continuous changes of the mitral and tricuspid annuli during the cardiac cycle. When a rigid ring of the type utilized by Carpentier is utilized, the annulus is not permitted to undergo these changes. Another disadvantage with the rigid ring is the tendency for the securement sutures to be torn loose from the annulus as the result of stress caused by restraining the annulus from undergoing normal physiological changes during the cardiac cycle.
Thus, Duran and Ubago considered it desirable to construct a flexible ring for use in atrioventricular annuloplasty so as to accommodate and follow the movements of the annulus during the cardiac cycle. It was their belief that in addition to the obvious advantage of allowing the heart to function in a more natural manner, use of a flexible ring would also decrease the chance of having the ring become partially detached from the annulus, because the stress forces on any particular point along the ring are reduced. Thus, Duran and Ubago developed a totally flexible prosthetic ring. Their initial results are reported in The Annals of Thoracic Surgery, Volume 22, No. 5, November 1976.
Subsequently, Massana improved upon the totally flexible ring by the addition of a traction thread which is passed through the interior of the ring, with both ends exiting the ring a short distance apart. Once implanted, the ring can be reduced in size, together with the annulus, by pulling on the traction threads until the valve becomes competent in a manner very similar to pulling a pair of purse strings. The ring may be contracted symmetrically by pulling both ends of the traction thread the same amount, or the ring may be contracted asymmetrically by pulling one end of the traction thread more than the other. Once the desired conformation of the valve is achieved, the ends of the traction thread are tied off. Massana's device is described in Cardiovascular Surgery 1980, a publication of Springer-Verlag Berlin Heidelberg.
Unfortunately, the totally flexible ring of Duran et al. and the totally flexible ring as modified by Massana, both fail to restore normal heart valve function. As discussed above, the most common heart valve defect is a dilatation of the posterior two-thirds of the valve annulus and an accompanying loss of normal configuration of the valve. Thus, the natural tendency of the damaged valve is to assume an unnatural shape. The rings of Duran et al. and Massana are unsatisfactory because they allow too much movement of the valve annulus, and the "normal" movement of a dilatated annulus is away from proper function.
Thus, it would be a significant contribution to the art of atrioventricular annuloplasty to provide a valve prosthesis that allows desirable anatomical movements of the annulus during the cardiac cycle, yet maintains a damaged annulus in the proper physiological shape to insure proper functioning of the valve.